Pickling is the process of chemically removing oxides and scale from the surface of a metal by the action of water solutions of inorganic acids. The reaction occurring when steel or iron materials are immersed in dilute inorganic acid solutions includes the solution of metal as a salt of the acid and the evolution of hydrogen. Steel pickled in dilute hydrochloric acid and sulfuric-acid solutions is an example of this reaction, with the end products of reaction being, respectively, ferrous chloride and hydrogen and ferrous sulfate and hydrogen. Adherent films of oxides are removed by the acid attack upon the scale on the base metal.
The pickling zone for pickling metal usually consists of several individual acid-proof tanks, typically about two to three feet in depth, located in series, comprising an effective immersion length of about 250 to 300 feet. While many lines have from three to five tanks, each about 40 to 80 feet long, some lines have only one long tank, divided by weirs into four or five sections. The strip is completely submerged under several inches of liquid acid bath as it travels through the tank or series of tanks forming the pickling zone.
The rate of pickling metal in the pickling zone is affected by numerous variables, including the metal-based constituents and the type and adherence of oxide to be removed. Solution temperature and concentration, ferrous chloride or ferrous sulfate concentration, agitation, time of immersion and presence of inhibitors all influence the rate of acid attack. Rate of acid attack and tank efficiency are important considerations when choosing operational parameters such as the choice of acid, hydrochloric acid or sulfuric acid for example, and the temperature of the acid bath. One of the most common efficiency problems in today's pickling systems is heat loss from the acid baths. The loss of heat lowers the efficiency of the overall system and will be more costly to operate.
Much of the heat supplied to the acid baths in pickling operations is lost at the surface of the acid bath by liquid evaporation. The heated acid bath solution evaporates into the air space between the acid bath and a cover enclosing the tank from the outside. In conventional pickling systems, the space above the acid bath and below the tank cover is used for transporting exhaust gases, evaporated acid bath fumes and air, to exhaust exits connected to the tank cover. Thus, the exhaust gasses in this space are not static, but form a fluid stream that typically flows in one direction, discharging through the exhaust exits. The contact of this fluid stream, traveling just above the acid bath surface, with the bath increases the evaporation rate at the air-liquid interface of the bath.
Increased evaporation leads to increased heat loss which means that more heat must be supplied to the acid baths in order to maintain the acid baths at a constant temperature. Maintaining the acid baths at a constant temperature is important in achieving even pickling of a length of metal product. By maintaining a specific acid temperature, the pickling process can either be sped up or slowed down to achieve the proper pickling of a product. But heat loss from the acid bath can complicate the pickling process by creating temperature gradients in the acid baths which can cause uneven pickling of the metal product.
In many present designs, the surface of the acid bath is open to the circulation exhaust gases from the pickling system. This facilitates heat loss. Because most of the heat loss of the acid is from the surface of the liquid in the form of evaporation, it is important to control this loss.